DESCRIPTION (Applicant's Abstract): This proposal addresses the problem of evaluating the efficacy of newly developed agents for the treatment of primary pulmonary hypertension. The use of catheter techniques to measure pulmonary vascular resistance severely limits routine evaluation of such treatments. We propose to develop, refine and test a non-invasive ultrasound based means of accurately evaluating pulmonary vascular resistance in children with primary pulmonary hypertension. The hypothesis for this project is based on the relationship between changes in downstream impedance within a fluid system and the characteristics of the pressure pulse propagation wave that develops within the arterial walls. We propose to show that downstream impedance affects the pulse propagation wave traveling within the main pulmonary artery and that changes in downstream impedance, as would occur with treatments such as inhaled nitric oxide or infused prostaclyin, can be followed by measuring pulse propagation characteristics. Furthermore, we propose that the pressure pulse propagation in the main PA affects local velocities, and that such changes in local velocities can be quantified as a velocity propagation using non-invasive ultrasound color M-mode imaging. This should significantly aid in evaluating new treatments for primary pulmonary hypertension and thereby expand treatment options and improve quality of life for patients. The aims of this project, therefore, are: 1. Demonstrate analytically that a fundamentally rooted mathematical and physical foundation exists for using velocity data to extract pressure pulse propagation characteristics for pediatric primary pulmonary hypertension. 2. Develop and test a method for using color M-mode velocity data to predict downstream impedance using highly reproducible in vitro models. 3. Determine clinical utility of the color M-mode approach using existing clinical protocols studying the efficacy of nitric oxide and/or 100 percent 02 treatment in the catheterization laboratory to reduce pulmonary vascular resistance in children with primary pulmonary hypertension. 4. Determine whether color M-mode measured velocity propagation (Vel-prop) predicts pulmonary vascular resistance in the clinical situation.